One result of the miniaturization of circuitry is that many more circuits, and different types of circuits, can be placed in close proximity to one another. For example, in some consumer goods, in particular handheld radiotelephones such as cellular telephones and personal communicators, one may find a microprocessor, a digital signal processor, a memory and sensitive RF transmit and receive circuitry all located upon a single PCB within an area of but a few square inches. It can be appreciated that to ensure reliable operation it is important to reduce or eliminate cross-coupling and interference between these various circuits, as well as to prevent externally generated interference from coupling through the radiotelephone housing into the circuits and, vice versa, to prevent signals from the circuits from coupling through the housing to external circuitry.
It is well known in the art to provide a plastic housing having a metalized inner surface to reduce the passage of undesirable signals through a housing. However, metalized housings are expensive to fabricate. Furthermore, a metalized plastic housing is not as easily recycled as an unmetalized, plastic-only housing.
It is also well known in the art to provide metal enclosures for placing over certain sensitive circuitry to prevent signals from coupling into and/or out of the circuitry. For example, sensitive receiver circuits may have a metal shield (or "can") placed over them, with the can being held in place on the PCB with fasteners (e.g., screws) or soldered into place. Some means is then also provided for electrically coupling the metal shield to a common potential, such as circuit ground or RF ground.
Referring to FIG. 1, one prior art metal shield is placed over a sensitive integrated circuit (IC) 1, and includes an inner sheet metal shield forming a sort of wall around the IC 1. The inner shield 2 has a plurality of feet or flanges 3 that are held in place with solder 4 to a conductive trace 5A (e.g., a circuit ground trace) that runs over a surface of a PCB 5B. An outer sheet metal shield 6 or cover, which may be perforated, fits over the inner shield 2 and is connected to the inner shield 2 using some type of fasteners 7, such as spot welds or rivets. For ease of handling, the inner and outer shields can be connected together before being placed on the PCB 5B and soldered (e.g., using surface mount technology) to the trace(s) 5A.
As can be appreciated, the use of this type of metal shield (which may have dimensions of only two or three centimeters along a side) has a number of disadvantages, including high cost and weight (relative to a metalized cover), as well a requirement that expensive tooling be provided.
Furthermore, the use of sheet metal places restrictions on the shapes that the metal shield can assume. Also, the use of the metal shield may have an impact on the soldering operation, due at least to the relatively larger thermal mass of the shield as compared to the surface mount solder leads found on conventional ICs and other components.
Another conventional shielding technique uses a conductive compression gasket, which may be dispensed, die-cut or metal. However, this approach is also expensive, as well as being environmentally unfriendly and logistically complex, as well as having a high tooling cost.